1. Field of the Invention
The present invention relates to an air flow amount measuring apparatus for measuring flow amount of air flowing in an air flow passage, which may be applied to an intake system of an internal combustion engine, for instance.
2. Related Art
Conventionally, thermal type air flow amount measuring apparatuses (air flow meters) are known as disclosed in Japanese Utility Model Publications No. 6-23942 and No. 5-24189.
In the air flow meter disclosed in Japanese Utility Model Publication No. 6-23942, a convex part is formed at the upstream side of a heater wire and a step is formed at the downstream-side terminal end of the convex part. This step at the downstream-side terminal end disturbs the flow of air and generates periodical vortexes. A thermal sensing unit responds to periodical variations in the flow speed caused by the vortexes, resulting in possible detection errors due to periodical variation.
In the airflow meter disclosed in Japanese Utility Model Publication No. 5-24189, a pair of dust-proofing members are provided for effectively restricting oil and dust from adhering on electrical leads of a hot film type sensing unit. Those dust-proofing members stabilize accuracy in measurement of the air flow amount by suppressing changes in the heat radiation characteristics of the sensing unit caused by an accumulation of dust.